Abstract: Neurons grow elaborate structures tailored to send and receive electrical signals over large distances and through complex networks of connections. Neuronal morphology in these networks changes dynamically in response to both neuronal firing and external growth cues, during development as well as learning and memory, and recedes in the absence of positive growth cues. These growth cues are transduced by receptors that exhibit subcellular location-dependent signaling properties as they are internalized from the plasma membrane and trafficked through endosomal compartments, and therefore regulation of membrane traffic can profoundly alter growth signaling. The intersection of signaling and membrane traffic is particularly intriguing in the presynaptic compartment of neurons, because synapses are highly specialized for both exocytic and endocytic traffic of synaptic vesicles in response to activity. Signaling receptor internalization and the synaptic vesicle cycle use a highly overlapping set of trafficking machinery, but little is understood about cross-talk between these processes and how activity-dependent modes of regulation of trafficking machinery might be used to control signal transduction. These membrane trafficking events are deeply implicated in neurodegenerative diseases, and understanding how they are regulated by neuron-specific mechanisms may lead to new routes for intervention. We are using the Drosophila larval neuromuscular junction (NMJ) to unravel the molecular mechanisms by which activity-dependent changes in receptor traffic lead to changes in synaptic growth signaling and synaptic architecture. This proposal combines two approaches to determining how receptor traffic in this system is regulated by activity: (1) live imaging of signaling receptor traffic and in response to chronic and acute changes in activity, and (2) proteomic analyses of activity-dependent changes in the membrane traffic machinery. We will then combine our findings about molecular mechanisms of activity-dependent regulation of endocytic traffic with in vivo tests of traffic of synaptic growth receptor, leading to models for how activity works through the membrane traffic machinery to tune signaling up or down for structural remodeling of neurons, and for how mis-regulation of these cellular trafficking events might contribute to neurodegenerative disease. Public Health Relevance: Neurons undergo dynamic structural changes in response to external growth signals during development as well as learning and memory. Growth signals are trafficked into the cell via membrane-bound compartments, and defects in these events are a hallmark of neurodegenerative diseases such as Amyotrophic Lateral Sclerosis (ALS) and Alzheimer's disease. We propose to investigate how neurons control the formation of membrane compartments and load them with specific signaling cargoes, and identify ways to manipulate these events to treat neurological disease.